fix spelling, fix #17

Author: shawntabrizi

steps/8/README.md

@@ -12,7 +12,7 @@ Before we continue, let's take a moment to go over some Rust which we will be us
 
 ### Option and Option Handling
 
-One of the key principals of Rust is to remove undefined behavior from your code.
+One of the key principles of Rust is to remove undefined behavior from your code.
 
 One way undefined behavior can happen is by allowing states like `null` to exist. Rust prevents this by having the user explicitly handle all cases, and this is where the creation of the `Option` type comes in. Spend a moment to re-review [the section on `Option`](https://doc.rust-lang.org/book/ch06-01-defining-an-enum.html?highlight=option#the-option-enum-and-its-advantages-over-null-values) from the Rust book if needed.